The efficiency of a conventional switched power converter, such as a buck converter, is dominated by the losses within the switches (e.g. field effect transistors, FETs) and the inductor of the power converter. If the power converter provides a relatively high output voltage Vout, the power converter typically exhibits a reduced conversion efficiency, because the switches must be implemented in high voltage technology, and thus the switches have an increased switch area and increased reverse recovery losses. Relatively large FETs typically cause relatively high switching losses, because of an increased gate charge and LX capacitance.
The voltage which is applied to the inductor of a boost converter is proportional to the difference between the input voltage Vin and the output voltage Vout, i.e. Vin−Vout, during the magnetization phase, or proportional to Vout during the demagnetization phase. Increased inductor voltages cause increased current variations dI/dt and thus an increased switching frequency (for achieving a pre-determined current ripple) and/or an increased current ripple (for a given switching frequency). In both cases this leads to increased inductor core losses and to an increased dissipation power.
Maintaining low current variations dIL/dt at increased input and output voltages typically requires inductors (i.e. coils) with increased inductance L, due to the relation dIL/dt=VL/L. However, coils with an increased inductance L have an increased number of turns. For inductors to maintain their Direct Current Resistance (DCR) even with an increased number of turns, each turn has to make use of a wire with an increased thickness to compensate for the impedance increase. Thus the size of the inductor is growing twice with an increased inductance L (due to the increased number of turns and due to the increased wire thickness). On the other hand, if the inductor dimensions are not increased, an increased inductance L leads to the effect that the DCR of the inductor is growing twice due to the additional number of turns and due to the use of a thinner wire.
Over the last years battery powered applications such as smartphones and tablets exhibit increased LCD resolution and size, therefore demanding for LED backlight with higher brightness and size. As a result of this development, the power which is required for LED backlight has increased, despite the fact that LEDs have improved efficiency. As LED brightness is proportional to its current, a uniform LED backlight brightness may be achieved by connecting multiple LEDs in series (typically 6-7 LEDs). A boost converter may be used to drive such LED strings, because the typical supply voltage for such LED strings (16-20V) is substantially higher than the output voltage of a LiIon battery pack (˜3.7V).
As backlight power is required most of the time for an active application, the boost converter efficiency contributes significantly to the overall mobility time of a portable application. Smartphones are space and height constrained (especially regarding the inductors used for switching converters). Consequently, the boost converter cannot use coils of high inductance L. As a result, either the DCR of the coil or the switching frequency may need to be increased. Both measures lead to reduced efficiency.